Object about ill-considered heavy defeat (9)
I believe the answer is:
thrashing
'heavy defeat' is the definition.
(a decisive or devastating defeat)
'object about ill-considered' is the wordplay.
'object' becomes 'thing'.
'about' means one lot of letters goes inside another.
'ill considered' becomes 'rash' (synonyms).
'thing' placed around 'rash' is 'THRASHING'.
(Other definitions for thrashing that I've seen before include "beating" , "Hiding" , "A very heady defeat" , "punishment" .)